1. Field of the Invention
The present invention relates to a method of preparing a transparent polymer material comprising a thermoplastic polycarbonate and surface-modified mineral nanoparticles, as well as to a transparent polymer material obtained by said method.
The invention typically, but not exclusively, applies to the fields of optics, in particular optical articles of the automobile glass type, and to optical articles of the sighting instrument lens type, of the helmet visor type, or of the ophthalmic lens type.
More particularly, the term “automobile glass” not only means external transparent bodywork elements such as rear lights, side panels, side windows, glazed roofs, headlight or sidelight glazing, but also transparent elements for the interior, such as dashboard, dial, or screen glazing.
The term “ophthalmic lens” means lenses that are in particular suitable for mounting in eyeglasses, having the function of protecting the eye and/or correcting vision, such lenses being selected from afocal, unifocal, bifocal, trifocal and progressive lenses.
2. Description of Related Art
Polycarbonate enjoys advantages that render it particularly advantageous for optics, in particular excellent transparency, excellent shock resistance, a high refractive index and very light weight. In contrast, its principal disadvantages lie in it not being very rigid and being sensitive to scratching and abrasion.
In order to improve the mechanical properties of a polymer, in particular rigidity and abrasion and scratch resistance, it is known to add mineral nanoparticles to the polymer.
Typically, said mineral nanoparticles are incorporated directly into the polymer in the molten state.
However, the nanometric dimension of the mineral nanoparticles inevitably introduces a phenomenon of aggregation of said nanoparticles while being mixed into a thermoplastic polycarbonate matrix in the molten state.
For this reason, the polymer material obtained using that method loses transparency and also suffers coloration, in particular yellowing, rendering it difficult to use in optical fields.
Furthermore, incorporating mineral nanoparticles may induce degradation of the mechanical properties of the polymer material such as its shock resistance, for example.
Thus, in order to improve the quality of the interface between the nanoparticles and the thermoplastic polycarbonate matrix and to thereby improve the mechanical and optical properties of the polymer material, it is known from document EP-1 767 562 to use a method of preparing transparent polymer materials comprising a thermoplastic polycarbonate and surface-modified mineral nanoparticles.
That method consists in grafting a monomer for the thermoplastic polycarbonate matrix onto the surface of the mineral nanoparticles via an aliphatic ether bond, and then polymerizing said monomer in situ to form the thermoplastic polycarbonate matrix.
Thus, modification of the surface of said mineral nanoparticles cannot be dissociated from the thermoplastic polycarbonate matrix.
As a consequence, that prior art preparation method is industrially limited since only polycarbonate can be used to modify the surface of said mineral nanoparticles.
Furthermore, that method involves the use of a specific reactor to polymerize the thermoplastic polycarbonate matrix; that use is relatively restrictive.